Anodic production of pigmented siliceous coatings for aluminous metals

ABSTRACT

PIGMENTED SILICEOUS COATINGS HAVING GOOD ADHESION, HARDNESS AND SMOOTHNESS ARE PRODUCED ON THE SURFACE OF AN ALUMINOUS METAL BY IMMERISING THE METAL AS ANODE IN AN AQUEOUS BATH OF AN ALKALI METAL SILICATE CONTAINING AN INSOLUBLE PIGMENT, AND PASSING FULL-WAVE RECTIFIED ALTERNATING CURRENT THROUGH SAID BATH AT A VOLTAGE SUFFICIENT TO CAUSE DISSOCIATION OF THE BATH CONSTITUENTS BY MULTIPLE SPARK DISCHARGING AT THE ANODE SURFACE. THE BATH ALSO CONTAINS AN ALKALI CHROMATE, MOLYBDATE OR TUNGSTATE, AND AN ALKALI CARBONATE, PHOSPHATE OR BORATE. THE COLORED COATING THUS OBTAINED IS DECORATIVE AND USEFUL ON BUILDING TRIM, ALUMINUM SHINGLES AND SIDING, AND THE LIKE.

3 812 023 ANODIC PRODUCTION OF PIGMENTED SILI- CEOUS COATINGS FOR ALUMINOUS METALS Daniel J. Schardein, Jr., and Charles M. Rogers, Richmond, Va., and H. Lee Craig, In, Miami, Fla., assignors to Reynolds Metals Company, Richmond, Va. No Drawing. Filed Dec. 11, 1972, Ser. No. 314,045

Int. Cl. C231) 9/02 US. Cl. 204-58 6 Claims ABSTRACT OF THE DISCLOSURE trim, aluminum shingles and siding, and the like.

BACKGROUND OF THE INVENTION The present invention relates to an improved method of obtaining colored coatings on aluminous metals.

The coating of aluminum and its alloys in order to obtain improved corrosion and abrasion resistance and to impart color to the metal surface is well known in the prior art. One type of finish for aluminum is the electrolytic oxide coating produced by treating the metal as anode in various electrolytes to obtain a thin, inert and durable aluminum oxide film on the metal surface. This process, known as anodizing, may be accompanied by coloration of the coating by including color forming substances, usuallly soluble therein, in the bath. The anodic coatings thus produced may be clear and transparent or they may be subsequently colored by the application thereto of organic dyes or inorganic pigments.

More recently, proposals have been made to coat aluminum and other metals by deposition of a film thereon, using the metal as anode in a bath of sodium silicate, aluminate, or tungstate, employing an anodic spark reaction wherein the voltage applied to the system is sufiicient to cause sparking at the anode surface. The coatings thus produced are generally light in color and quite hard. They have the disadvantage, however, of requiring rather high voltages for application, of the order of 400-600 volts DC, and their field of usefulness is limited by their rough surfaces and their limited adhesion to the aluminous metal.

BRIEF SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a novel pigmented alkali silicate system and method for the production of colored coatings on aluminousmetals. The term aluminous metal as employed herein means aluminum and aluminum base alloys.

The present invention concerns the treatment of aluminous metal or an article made therefrom, to deposit electrokinetically an adherent colored siliceous coating on the metal surface, by immersing the metal as anode in a pigmented aqueous bath of an alkali silicate and passing an electric current through the bath accompanied by uniform spark discharge at the anode surface. The resulting colored coated articles are used for the production of architectural trim, siding, shingles, and in other ornamental applications of aluminum. They have a matte,

opaque appearance, and a color which depends upon the 3,812,023 Patented May 21, 1974 ice type of pigment employed. The coatings are composed principally of silica, with particles of pigment, and smaller amounts of alumina and of the alkali metal of the silicate bath.

These coatings may be treated or sealed subsequently with organic or inorganic protective materials, such as alkali metal silicates which can be baked or cured to increase their exposure resistance, or a wax, or a plastic or other polymer similarly affording a transparent protective layer.

DETAILED DESCRIPTION OF THE INVENTION In the electrokinetic type of deposition employed in accordance with the present invention, the aluminous metal as the anode is subjected to electrolytic treatment in an aqueous alkali silicate solution under conditions and at a voltage sufficient to cause a uniform sparking or are discharge at the anode surface. This is achieved by control of the composition of the alkali silicate electrolyte, and by employing a full-wave rectified alternating current having a maximum voltage of the order of about 250 to 350 volts.

The alkali silicate employed may be, for example, potassium or sodium silicate. As available commercially, these substances have the general form K O-xSiO or Na O-xSiO with widely varying ratios of SiO to K 0 or Na O. In accordance with the invention, there is employed an alkali silicate having a relatively high ratio of SiO; to K 0 or Na O, ranging from about 4:1 to 2:1.

The preferred alkali silicate for the practice of the invention is potassium silicate having a molar ratio of SiO K 0 between about 2.021 and 3.45:1. The practice of the invention will be described with respect to the use of potassium silicate, but it is to be understood that this is for purpose of illustration only, and is not to be considered as limiting.

While potassium silicate may be prepared having any predetermined SiO /K O ratio, it is preferred to employ as a source thereof a commercially available aqueoussolution sold under the designation Kasil No. 88 (Philadelphia Quartz Co., Philadelphia, Pa.) which has a SiO K 0 mol ratio of 3.45 and contains 19.9% SiO by weight. This product contains about 30% by weight of solids, has a pH of about 12, and is as viscous as a light oil. The concentration of Kasil No. 88 may be adjusted by dilution with water between practical limits of about 1 volume Kasil to 30 volumes water (0.8% Si0 by weight), to about 1 volume Kasil to 6 volumes of water (3.5% by weight SiO Typically the dilution is about 1:15 (1.5 weight percent SiO Too great a concentration of SiO' produces a nonuniform coating wtih loose particles about the surface. Too dilute a solution produces a thin, poorly distributed coating. T 00 high a ratio of SiO- /K O results in an unstable solution which tends to precipitate silica.

' This can be adjusted by addition of K 0 in the form of KOH solution, using a solution of about 45% KOH by weight.

As pigments, there may be employed in accordance wth the invention, one or more of a wide variety of inorganic or organic coloring materials, which are not soluble in nor adversely affected by the alkaline alkali silicate solution. These include pigments which ordinarily impart color bythe absorption of visible light, as well as luminescent (phosphorescent) pigments which emit colored light, under suitable excitation such as ultraviolet light.

The concentration of the pigment is not critical, the lower limit of concentration in the electrolyte depending upon the degree of pigmentation desired in the finished coating. Thus, if a very light coloration is desired only a very small amount of pigment in the silicate solution is needed. The upper limit on pigment concentration is a practical value indicated by the point at which further pigment additions do not result in further pigmentation of the silica coating. However, a very high concentration of pigment does not adverselyalfect the silica coating.

The pigment particle size range will depend upon the type of pigment employed, which may vary from colloidal pigments to commercial paint pigment dispersions. If necessary a suitable dispersing agent can be added to obtain requisite dispersibility, such as, for example sodium hexametaphosphate, tetrasodium pyrophosphate, trisodium phosphate, or sodium tripolyphosphate, in an amount between about 0.01% and about 0.5% by-weight of the bath. The pigment particle size range will ordinarily be of the order of .001 to 10 microns, and not greater than 30 microns. Agitation of the bath may be required to maintain pigment dispersion in the bath, unless pigment particles of a colloidal size are involved.

The colors contemplated by the invention range from white to black, with intermediate reds, oranges, greens, and blues. The inorganic pigments yielding white coatings include: various metal oxides, such as zirconium dioxide and zinc oxide. Other inorganic metal oxides which may be used include those yielding red colors, such as ferric oxide and cuprous oxide. Metal oxides imparting grey colors to the silica coating include graphite, carbon, lead monoxide, iron oxide (FeO), manganese dioxide, titanium dioxide, antimony trioxide, and nickel oxide. range shades may be obtained by use of, for example, red lead, or cadmium oxide. Yellow shades are obtained by use of chrome yellow (lead chromate). Greens are obtained with silver oxide or chrome green (CD 0 Blue shades result from the use of ultramarine blue. Black shades are provided by the use of graphite suspensions, carbon black, lamp black, or ferrous oxide FeO.

Examples of pigments providing a phosphorescent coating include zinc sulfide or cadmium sulfide or a combination thereof, advantageously with either silver or copper as an activator.

Where metal oxide pigments are utilized, an advantageous approach is to add approximately 15 grams of oxide per liter of bath, employing Kasil No. 88 at dilution of 1:15.

In order to obtain a smoother, harder and more ad herent coating, there may be incorporated in the alkali silicate bath a suitable amount of an alkali chromate, molybdate, or tungstate, or mixtures thereof. Preferably the alkali is the same as that of the silicate. Examples of suitable additives are the potassium or sodium salts, preferably the potassium salts such as potassium chromate K CrO potassium molybdate K Mo0 or potassium tungstate K WO These compounds are added in amounts sufiicient to achieve a total concentration in the diluted aqueous silicate bath between about 0.005 and about 0.1 molar, preferably between about .01 and about .05 molar.

Further, in accordance with the invention there may also be incorporated in the alkali silicate bath a suitable amount of an alkali carbonate, sulphate, or borate, the alkali again being the sme as that of the silicate, such as potassium or sodium. The potassium salts are preferred, such as, for example, potassium carbonate K CO potassium orthophosphate K PO or potassium tetraborate K2B407. These salt additives serve to improve the bath conductivity and the coating smoothness, and are present in the general concentration range of about 0.01 to about 0.1 molar, preferably about .03 to about .05 molar. Thus, for example, 1 liter of Kasil No. 88 containing 4.17 mols SiO is diluted with 15 liters of water, and potassium tetraborate added thereto in an amount such that the addition is equivalent to 0.1 mol B 0 Bath temperatures may lie between room temperature and about 100 C. Too high a temperature tends to reduce film thickness. Coating time required may range from less than one minute to about one hour, typically between about two and about five minutes.

Coating thicknesses of from about 0.10 up to 3.5 mils can be obtained, with thicknesses of 0.15 to 0.5 being typical. The coating thickness is dependent upon the SiO; content of the bath, with from about 0.5% to about 9% SiO (at a mol ratio of SiO /K 0 of 3.45) or it equivalent, being preferred.

If filtered and ripple-free direct current is employed for deposition, it is diflicult to prevent isolated burning or arcing at the anode, which results in formation of a. thick crusty mass of fused silicate glass having poor adhesion. The use of alternating current alone, and without :modification, tends to cause anodic oxidation of the aluminous metal during the positive phase, no deposition of silicate during the zero voltage interval, and formation of hydrogen at the maximum negative phase, resulting in gaseous displacement of the coating.

Hence, for purposes of the present invention, the power source employed is preferably a pulsed DC. current, such as an unfiltered full-wave rectified alternating current. The wave form is modified by the introduction of suitable rectifying means on the supply side of the circuit, such as a silicon rectifier, diode or mercury arc rectifier. In this way, there are supplied to the alkali silicate electrolyte bath and the electrodes, pulses of current at the desired predominantly positive voltage. This avoids harmful negative voltages and the danger of removal of portions of the coating through generation of hydrogen.

The use of controlled composition of the electrolyte and of rectified alternating current, as described, results in uniform spark discharge and the deposition of uniform, colored, opaque coatings having a matte appearance and having good adhesion to the aluminum substrate.

The choice of a suitable power supply will depend upon the maximum area of the metal or article being coated, as well as on the bath characteristics. There is a minimum current density below which coating formation does not take place, but above this minimum, current density is not critical, and may range from about 30 and about amperes per square foot. The voltage range is advantageously between about 250 and about 350 volts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples illustrate the practice of the invention, but are not to be regarded as limiting.

Example 1.--Black coating A 60 liter bath containing 1 part by volume of Kasil No. 88 (potassium silicate) and 20 parts by volume of water is prepared. As a black pigment there is incorporated with the potassium silicate solution 12 grams per liter of Dag 137, a concentrated colloidal dispersion (22% solids) of pure electric furnace graphite in a liquid vehicle, equivalent to 2.6 g.p.l. of colloidal graphite. The Slog/K20 mole ratio of the solution is 3.45:1. 2" x 2" coupons of aluminum alloy 7-16 suspended in the bath.

No agitation is needed to maintain pigment dispersion. Full-wave rectified alternating current was passed thru the bath, the aluminum alloy being the anode at a voltage between 250 and 350 volts, at a current density of 36 amperes per square foot, for 3 minutes, at room temperature. A black coating having a thickness of 0.3 mil was obtained.

Example 2.--'Green coating Proceeding as in Example 1, a bath of Kasil No. 88 was prepared in a dilution of 1:15 with water, by volume. There were included in the bath 0.04 M potassium carbonate and 0.02 M potassium chromate. Chromium green oxide pigment was suspended in the bath in the amount of 15 g.p.l. The; Slog/K30 mol ratio was bath. W maintained at room temperature.

The current was passed thru the system at a voltage between 250 and 310 volts, and at a current density of 60 amperes per square foot for 3 minutes. A green coating having a thickness of 0.6 mil was produced.

Example 3.Phosphorescent coating Proceeding as in Example 1, a bath of Kasil No. 88 was prepared in a dilution with water, by volume, of 1:9. Phosphorescent zinc sulfide pigment was incorporated in the bath in an amount of 15 g.p.l. The SiO /K O mol ratio was 3.45:1. Current was passed thru the system at a voltage of 2-60 to 350 volts, as a current density of 36 amperes per square foot, for 4 minutes, at room temperature. A white coating of 0.7 mil thickness was produced.

Example 4.Coated shingle stock Proceeding as in Example 1, a 2" x 2" coupon of aluminum shingle sheet stock was employed. A slurry of iron oxide pigment in water was prepared so as to insure maximum wetting of the pigment particles, water suflicient to dilute the slurry to /z the required volume was added, then Kasil No. 88 was added, and finally sufiicient water to result in a Kasil dilution of 1:10 by volume. Using a current density of 108 amperes per square foot, and 250 volts, a coating having an attractive earthtone shade was obtained. The adhesion is improved by incorporating in the bath 0.02 M potassium carbonate and 0.04 M potassium chromate.

What is claimed is:

1. Method of electrokinetically producing a pigmented siliceous coating on an aluminous metal surface, comprising the steps of:

(a) immersing the metal as anode in-an aqueous bath consisting essentially of (i) an alkali silicate,

(ii) a first salt selected from the group consisting of an alkali chromate, an alkali molybdate and an alkali tungstate, at a concentration between about .005 and about 0.1 molar, and

(iii) a second salt selected from the group consisting of an alkali carbonate, an alkali phosphate and an alkali borate, at a concentration between about .01 and about 0.1 molar,

said bath having an insoluble pigment dispersed therein, and

(b) passing current through said metal and bath at a voltage predominantly positive and sutficient to cause multiple spark discharge uniformly distributed over the anode surface and effect formation of said pigmented siliceous coating.

2. The method of claim 1 in which the alkali silicate is potassium silicate.

3. The method of claim 2 in which the potassium silicate has a mol ratio of SiO /K O between about 4:1 and about 2:1.

4. The method of claim 1 in which the pigment is a metal oxide pigment.

5. The method of claim 1 in which the concentration of alkali silicate in said bath is between about 0.8% and about 3.5% by weight of equivalent SiO;.

6. The method of claim 1 in which full-wave rectified current is used at voltage between about 250 and about 350 volts.

References Cited UNITED STATES PATENTS 3,658,662 4/1972 Casson et a1. 204-58 3,293,158 12/1966 McNeill et al. 204-58 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner 

